The invention relates to a fluidized-bed reactor suited for polymerization or copolymerization of olefins. The invention particularly concerns a fluidized-bed reactor having its bottom section equipped with means for uniform distribution of the circulation gas passed to the bottom of the reactor over the entire cross section of the reactor.
Fluidized-bed reactors are conventionally used in the continuous gas-phase polymerization of olefins into their homo- and copolymers. In a fluidized bed the polymerization reaction is carried out in a bed comprised of polymer particles formed during polymerization, whereby the bed is kept in the fluidized state with the help of a circulating gas flow directed upward from the bottom section of the reactor. The circulating gas flow includes both the gaseous monomers to be polymerized and inert gases and/or gaseous hydrocarbon diluents. The circulating gas flow is removed from the top of the reactor upper section, passed to heat exchangers for the recovery of heat produced by the polymerization reaction and finally returned to the bottom section of the reactor with the help of a compressor.
A uniform distribution of the circulating gas passed to the lower section of the reactor is mandatory for maintaining a steady state of fluidization. The distribution of the circulating gas is conventionally accomplished by means of a flow distributor element formed by a perforated intermediate dividing plate which is placed close to the bottom of the reactor vessel. Thus, an infeed/mixing chamber of the circulating gas is formed at the bottom section of the reactor separately from the fluidized bed layer, that is, the ordinary polymerization space.
The larger the reactor vessel volume, the more difficult it is to achieve uniform distribution of the circulating gas in the fluidized bed over the entire cross section of the reactor. Due to the friction caused by the wall to the flow, also very small reactors present similar problems in attempts to achieve uniform cross-sectional distribution of the flow. As a result of the non-uniform flow distribution, the fluidized bed gradually develops denser or less fluidized spots, particularly in the vicinity of the walls. This problem is worse when liquid fractions enter into the reactor along with the circulating gas. A uniform distribution of liquid phase components into the fluidized bed is particularly difficult to achieve. Consequently, the operation of the fluidized bed will deteriorate due to local overheating and agglomeration of polymer particles into larger clumps, as well as adherence of such agglomerates to the reactor walls.
To improve the uniformity of gas flow distribution, use of such gas distributor plates have been proposed in which the size, shape and location of the holes have been modified. However, the manufacture of such gas distributor plates is costly and their gas permeability may be inadequate thus causing unneeded pressure drops in the gas circulation through the reactor. Furthermore, accumulation of polymer clumps under the gas distributor plate is possible as the polymerizing particles entrained in the circulating gas flow may adhere to the wetted areas that can develop to the underside of the distributor plate.
Another common problem occurring with larger sizes of fluidized-bed reactors is the agglomeration of polymer particles and the adherence thereof to the wall surfaces of the reactor bottom section. This is because the circulating gas inevitably carries away from the reactor minor amounts of entrained, small polymer particles containing active catalyst that are then returned to the bottom section of the reactor in the circulating gas flow. If the circulating gas is passed to the reactor in the conventional manner via a straight tubular nozzle located at the reactor bottom and if the flow conditions or the shape of the reactor bottom are not optimal, accumulation of polymer particles within the feed chamber of the circulating gas begins. To eliminate this disadvantage, different types of flow distributor elements have been proposed for use connected to the circulating gas feed pipe. Accordingly, U.S. Pat. No. 4,877,587, for example, has at the reactor bottom the circulation gas feed pipe provided with distributor means which divide the flow exiting the pipe into two portions so that one portion of the flow is directed upward. In this case the division of the flow is implemented by means of a cap-shaped element having one large center opening on its upper surface and large openings on its sides. Hence, a portion of the flow is directed upward while the other portion is directed sideways. Such an embodiment has the disadvantage that the plain surfaces of the flow distributor element surrounding the circulating gas inlet flow opening tend to collect polymer particles from the circulating gas flow. Finally, large polymer clumps will grow on the flow distributor element. Also these embodiments cannot entirely prevent the occurrence of local vortexes in the bottom section of the reactor and resulting agglomeration and adherence of polymer particles on the wall surfaces. Removal of the polymer clumps and cleaning of distributor elements is clumsy and invariably requires reactor shutdown, opening and cleaning of its bottom section, which is an awkward and costly operation.
The typical shape for a fluidized-bed reactor comprises a more or less hemispherical surface similar to that described in cited U.S. Pat. No. 4,877,587. The benefit of such a shape of the bottom dome over a flat bottom is that the side of the reactor bottom section is free of any sharp-angled corners which might impede the gas flow and thus cause agglomeration of polymer particles. If the inlet arrangement of the circulating gas flow would be via a feed pipe or opening located at the center of the bottom, such an arrangement would fail in commercial reactors having a bottom diameter of up to several meters.